The present invention relates generally to scanning apparatus and, more specifically, to a mechanical apparatus for deflecting a light beam comprising a movable mirror, a reaction mass and associated driving system.
In scanning technology, it is desirable to have a scanning apparatus capable of high scanning accuracy and rapid scan rate for scanning a target area in a conical shaped pattern. In some applications such as spacecraft, it is also desirable to minimize both power consumption due to limited power available on the platform, and vibration so as not to perturb the platform. Additionally, it is important to minimize the size and weight of the scanning apparatus due to size and weight limitations of the spacecraft. Furthermore, it is desirable to have an apparatus which is simple to fabricate, thus reducing the associated time and cost of making the apparatus.
A present practice for producing conical motion of a beam of light at high frequency uses a particular type of crystal through which the beam of light is passed and across which high voltage electric fields are established in directions perpendicular to one another by external electrical circuits. The beam of light must be polarized in a particular direction and that direction must be maintained during operation. As the electric fields are varied in magnitude and changed in polarity, the light beam is deflected through varying angles. By varying the electric fields sinusoidally at the same frequency, and by controlling the relative phase angle of the two sinusoids, the beam as it exits the crystal is deflected in a conical pattern. The pattern""s cross section may be circular, elliptical, or linear depending on the relative phase angle between the two sinusoids. The voltages must have peak amplitudes of several hundred volts to produce even small deflections of the beam of light. While this practice allows the use of a relatively small and lightweight system which will not perturb the spacecraft, it requires high voltages and a significant amount of power to operate. Additionally, it requires the use of polarized light.
Another practice for scanning a beam of light is reflecting a light beam off of a mirror which is configured to oscillate, thus producing a reflected scanned beam. The mirror may be rotatably positioned on bearings and driven by a motor. While this can provide scanned beams which are conical in shape, the system may take a significant amount of power to operate, be costly to fabricate, and also transfer a torque to the platform on which it is mounted. Alternatively, the mirror may be connected to a reaction mass, with the reaction mass moved in an opposite direction of the mirror, thus reducing the torque transferred to the platform on which it is mounted. However, this type of system typically has the mirror and reaction mass coupled to one another at two opposite edges of the mirror, thus limiting the scanning to only linear scanning. Two mechanisms could be used in tandem to provide conical scanning, although this would be difficult to fabricate and may reduce the accuracy of the scanning and increase the power required to operate the system. These systems may also operate using a spring member and using mechanical resonance to reduce the amount of power to operate the system. Again however, this can make the system difficult to fabricate and also limit the system to linear scanning.
In accordance with the present invention, a high frequency mechanical scanning apparatus is disclosed that can reflect a beam directed toward its top surface in one or two dimensions, allowing linear, circular, and elliptical scanning of the reflected beam at a rapid scan rate. The scanning apparatus includes an upper resonant system which has an upper mass, an upper flexure and a reaction mass. The upper flexure is joined to the upper mass and the reaction mass, and has an associated first stiffness. A lower flexure is joined to the reaction mass and has an associated second stiffness. An excitation system is used to cause movement of at least the upper mass. The upper mass has a first width, the upper flexure has a second width and the lower flexure has a third width, with the second width being greater than the third width. The lower flexure is joined to the reaction mass in a recessed portion of the reaction mass. The upper mass, upper flexure, reaction mass and lower flexure are preferably an integral unit, which is attached to a base assembly at the lower portion of the lower flexure. The first width of the upper flexure and the second width of the lower flexure are chosen such that the second stiffness is lower than the first stiffness. In one embodiment, the second stiffness is no greater than about 10% of the first stiffness.
During operation, the upper mass deflects rotationally about an upper translational node and the reaction mass deflects rotationally about a lower translational node. The upper translational node is preferably as close as possible to the center of mass of the upper mass, and the lower translational node is preferably as close as possible to the center of mass of the reaction mass. Additionally, the lower flexure preferably has its center of rotation located adjacent to the lower translational node. The excitation system which creates the movement of the upper mass includes at least first and a second excitation magnets and a coil assembly which applies driving torques to the excitation magnets to sustain motion of the upper mass.
In one embodiment, the upper resonant system resonates about its upper and lower translational nodes. The resonance is maintained by the upper flexure acting as a spring in which energy is stored and released into the system. Thus, when the upper mass is moved from its undeflected position to a deflected position, kinetic energy is stored in the upper flexure, and as the upper mass is moved from a deflected position to an undeflected position, the stored energy is released back to at least the upper mass.
Based on the foregoing summary, a number of advantages of the present invention are noted. A mechanical scanning apparatus is provided that improves previously developed scanning apparatuses by allowing one or two dimensional scanning of a reflected beam. The reflected beam may be scanned linearly, or in an elliptical or circular pattern. The scanning apparatus operates at a resonance frequency, thus allowing it to maintain a rapid scan rate with reduced power needs. Fewer parts are required and assembly time and cost are reduced due to the highly integral nature of the design. Additionally, the apparatus transmits very little torque to the base assembly due to the stiffness and location of the connection point of the lower flexure.
Other features and advantages will be apparent from the following discussion, particularly when taken together with the accompanying drawings.